Field emitting devices are useful in a wide variety of applications, such as flat panel displays, microwave power amplifiers, and nano-fabrication tools. See U.S. Pat. No. 6,283,812 to Jin, et al. issued on Sep. 4, 2001 and entitled “Process for fabricating article comprising aligned truncated carbon nanotubes”. Also see U.S. Pat. No. 6,297,592 to Goren, et al., issued on Oct. 2, 2001 and entitled “Microwave vacuum tube device employing grid-modulated cold cathode source having nanotube emitters”.
A typical field emitting device comprises a field emitting assembly composed of a cathode and one or more field emitter tips. The device also typically includes a grid closely spaced to the emitter tips and an anode spaced further from the cathode. Voltage induces emission of electrons from the tips, through the grid, toward the anode.
Nanowires are potentially useful as electron emitters for field emission devices. Small diameter nanowires, such as carbon nanotubes with a diameter on the order of 1-100 nanometers, have received considerable attention in recent years. See Liu et al., SCIENCE, Vol. 280, p. 1253 (1998); Ren et al., SCIENCE, Vol. 282, p. 1105 (1998); Li et al., SCIENCE, Vol. 274, p. 1701 (1996); J. Tans et al., NATURE, Vol. 36, p. 474 (1997); Fan et al., SCIENCE, Vol. 283, p. 512 (1999); Bower et als., Applied Physics Letters, Vol. 77, p. 830 (2000), and Applied Physics Letters, Vol. 77, p. 2767 (2000). The term “nanowire” is used generically herein to include both solid nanowires and hollow nanowires (nanotubes).
Carbon nanotubes exhibit unique atomic arrangements, nano-scale structures, and unusual physical properties such as one-dimensional electrical behavior, quantum conductance, and ballistic transport characteristics. Carbon nanotubes are among the smallest dimensioned nanowire materials with generally high aspect ratio and small diameter, e.g., single-wall nanotubes may be made with diameters of ˜1 nm and multi-wall nanotubes with diameters of less than ˜50 nm.
High-quality single-wall carbon nanotubes are typically grown as randomly oriented, needle-like or spaghetti-like, tangled nanowires by laser ablation or arc techniques. Chemical vapor deposition (CVD) methods such as described above by Ren et al., Fan et al., Li et al., and Bower et al. produce multiwall nanowires attached to a substrate, often with a semi-aligned or aligned, parallel growth perpendicular to the substrate. As described in these articles, catalytic decomposition of hydrocarbon-containing precursors such as ethylene, methane, or benzene produces carbon nanotubes when the reaction parameters such as temperature, time, precursor concentration, flow rate, are optimized. Nucleation layers, such as thin coatings of Ni, Co, or Fe, are often intentionally added to the substrate surface to nucleate a multiplicity of isolated nanowires. Carbon nanotubes can also be nucleated and grown on a substrate without using such a metal nucleating layer, e.g., by using a hydrocarbon-containing precursor mixed with a chemical component such as ferrocene, (C5H5)2Fe which contains one or more catalytic metal atoms. During chemical vapor decomposition, the metal atoms serve to nucleate the nanotubes on the substrate surface. See Cheng et al., CHEM. PHYSICS LETTERS, Vol. 289, p. 602 (1998), and Andrews et al., CHEM. PHYSICS LETTERS, Vol. 303, p. 467 (1999).
Carbon nanotubes have been proposed for field emission devices such as flat panel field emission displays, microwave amplifiers and electron beam lithography devices. Conventional field emission cathode materials typically have been made of metal (such as Mo) or semiconductor material (such as Si) with sharp tips of submicron size. However, the control voltage required for emission is relatively high (around 100 V), because of high work functions and insufficiently sharp tips. To significantly enhance local fields and reduce the voltage requirement for emission, it would be advantageous to provide nanoscale cathodes with small diameters and sharp tips.
In field emission devices, unaligned, randomly distributed nanowires are inefficient electron emitters due to the varying distance and hence varying local electric fields between the cathode (comprised of emitting nanowire tips) and the gate or anode. In addition, when unaligned nanowires are used for emitters, an applied electric field between anode and cathode bends the nanowires along the field direction. The degree of bending is dependent on the applied voltage. This bending causes uncontrollable and undesirable changes in the distance between cathode and gate, and hence alters the local field on different nanowires. In some cases, the bending causes outright electrical shorting between the nanowire tips and the gate.
Referring to the drawings, FIGS. 1(a) and 1(b) (which are conventional) schematically illustrate configurations of aligned nanotubes 10 grown on a substrate 11 in a dense “forest-like” configuration (FIG. 1(a)) or in spaced-apart “forests” (FIG. 1(b)). A forest configuration, however, wastes the unique high-aspect-ratio, field-concentrating characteristics of individual nanowires.
Moreover, while the alignment of nanowires is important for many applications, highly oriented nanowires do not alone guarantee efficient field emission. The reason is that the individual nanowires are so closely spaced that they shield each other from effective field concentration at the ends. It is therefore desirable to create the spaced apart configurations of nanowires 10 schematically illustrated in FIGS. 1(c) and 1(d).
While it is desirable to provide a triode structure with nanowire emitters in the configurations of FIG. 1(c) or 1(d), it is difficult to do so. The fabrication of triode structures involves complex fabrication steps to produce a complex three dimensional structure of cathode, dielectric spacer and gate. Such fabrication often involves multilayer processing of silicon structures requiring various steps of patterning, lithography, and etching of silicon, silicon oxide, silicon nitride, and metal. The nanowires, if deposited before the fabrication steps, may not survive.
It is also difficult to add nanowire emitters after the three dimensional triode structure is formed. The insertion of a single nanowire nucleating nano-island (such as a 5-50 nm diameter island of cobalt or nickel) into the cavity below the suspended gate aperture would be difficult. The subsequent growth of nanowires by CVD is also difficult not only because of the high CVD processing temperature (typically higher than ˜550° C.) which can cause undesirable chemical diffusion, thermal stress build-up, and structural distortion of the three dimensional structure, but also because adequate CVD gas may not enter into the micro-size cavity in sufficient quantity or rate. In addition, the growth of an isolated nanowire or small group of nanowires along the vertical direction is difficult because of the absence of neighboring nanowires to provide mechanical support. Furthermore, it is difficult to centrally position the nanowire in the cavity underneath the gate aperture. Accordingly there is a need for improved triodes having centrally located nanowire emitters and methods for making such triodes.